Probing interactions of Ge with chemical and thermal SiO2to understand selective growth of Ge on Si during molecular beam epitaxy

We have previously demonstrated that Ge selectively grows on Si over a SiO2 mask during molecular beam epitaxy. To determine the surface phenomena responsible for the selectivity, we probed the stability of SiO2 upon Ge exposure and Ge diffusion through thin SiO2, using X-ray photoelectron spectroscopy, ellipsometry, atomic force microscopy, transmission electron microscopy, and multiple internal reflection Fourier transform infrared spectroscopy. We observe that the consumption of SiO2 occurs only with chemical oxide upon Ge exposure via Ge + Si + 2SiO2 → GeO(g) + 3SiO(g) at the SiO2/Si interface where all three reactants are present. This erosion is initiated by the Ge diffusion only through thin chemical SiO2, and the diffusion is attributed to a larger concentration of SiOH groups (∼1 × 1022 cm-3) and a greater porosity in chemical oxide than that in thermal oxide. For thermal SiO2, where Ge diffusion and subsequent oxide degradation are not observed, we have determined that the selectivity stems from...